In a helical intake port, a strong swirl effect is incompatible with a high intake efficiency. If an intake port of such a helical arrangement is used, capable of effecting a strong swirl effect for obtaining a stable operation during a low rotational speed condition of the engine, the intake efficiency becomes small during the high rotational speed condition of the engine, so that a large output of power cannot be produced. In addition, a strong swirl effect during the high rotational speed causes an excessive increase in the speed of the flow around the spark plugs, causing the mixture to become difficult to ignite. When the swirl effect is decreased to obtain a high power during the high rotational speed of the engine, combustion during the low rotational speed becomes unstable.